HR: 0800h
AN: GP11B-0827    [Abstracts]
TI: Paleomagnetism of Ar-Ar Dated Lava Flows From the Ceboruco-San Pedro Volcanic Field (Western Mexico): Evidences for the Matuyama-Brunhes Transition Precursor and a Fully Reversed Geomagnetic Event in the Brunhes Chron
AU: * Petronille, M
EM: petronil@clipper.ens.fr
AF: Ecole Normale Sup‚rieure de Paris, 45, rue d'Ulm, Paris, 75230 France
AU: Gogichaishvili, A
EM: avto@geofisica.unam.mx
AF: Instituto de Geofisica, UNAM, Ciudad Universitaria, Mexico, 04510 Mexico
AU: Henry, B
EM: henry@mailhost.ipgp.jussieu.fr
AF: IPGP, 4, Av. Neptune, Saint Maur, 94107 France
AU: Alva, L
EM: lalva@geofisica.unam.mx
AF: Instituto de Geofisica, UNAM, Ciudad Universitaria, Mexico, 04510 Mexico
AU: Rosas, J
EM: avto@geofisica.unam.mx
AF: Univ. Guadalajara, Centro de Ciencias de la Tierra, Guadalajara, 44840 Mexico
AU: Urrutia Fucugauchi, J
EM: juf@geofisica.unam.mx
AF: Instituto de Geofisica, UNAM, Ciudad Universitaria, Mexico, 04510 Mexico
AB: We report a detailed paleomagnetic and rock-magnetic study of seventeen independent lava flows belonging to the Trans-Mexican Volcanic Belt. 175 oriented samples were collected in the Ceboruco-San Pedro volcanic field. These sites were previously dated by means of the state-of-the-art 40Ar-39Ar geochronological method (Frey et al., GSA Bulletin, 116, 259-276, 2004) and span from 819 to 2 Ky. Rock-magnetic experiments which included continuous susceptibility and hysteresis measurements point to simple magnetic mineralogy. In most of cases, the remanence is carried by Ti-poor titanomagnetite of pseudo-single-domain magnetic structure. Fourteen flows give normal magnetic polarities while 2 are reversely magnetized and only one cooling unit yields intermediate paleodirections. The paleodirections of the flow dated as 819ñ25 ka correspond to a VGP latitude of 18øN. This anomalous field behavior apparently recorded prior to the Matuyama-Brunhes reversal may coincide with the transitionally magnetized lavas on La Palma, Canary Islands (40Ar-39Ar age - 822.2ñ8.7 ka, Singer et al., 2002) and with an event featured in several marine sediment records. Thus, this geomagnetic event, defined as M-B precursor, is probably global in extent. Two independent lava flows, dated as 623ñ91 and 614ñ16 ka respectively, yield reverse paleodirections. Age uncertainties make it extremely difficult to claim the discovery of a new geomagnetic event. It is possible that these lavas erupted during the worldwide observable Big Lost event (40Ar-39Ar age - 580.2ñ7.8 ka, Singer et al., 2002) which probably has been longer and more complex than it is generally believed for geomagnetic excursions. This study underlines the possible existence of a new, fully reversed geomagnetic event, but it is naturally premature to claim its presence in the Brunhes chron from this study alone.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1513 Geomagnetic excursions
DE: 1519 Magnetic mineralogy and petrology
DE: 1522 Paleomagnetic secular variation
DE: 1535 Reversals (process, timescale, magnetostratigraphy)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting